A plasma can be defined as an electrically conducting gas that satisfies quasi-neutrality. For singly charged ions, the type most often generated in ion and plasma sources, this means that the density of electrons and ions is approximately equal (ne≈ni). An ion or plasma source typically has a discharge region in which ions are generated by the collisions of energetic electrons with molecules of the working gas, a region of ion acceleration, and a region through which the beam of energetic ions travels after it leaves the source. Beams from industrial ion or plasma sources are used for etching, deposition and property modification. These sources operate in vacuum chambers, which are continually pumped while the source is operating to maintain a background pressure of approximately 10−3 Torr (0.13 Pascals) or less for ion sources and up to several times that high for some plasma sources. Ion or plasma sources are also used for space propulsion, in which case the beam provides propulsion for a spacecraft and the background pressure is much less than 10−3 Torr.
Both gridded and gridless ion and plasma sources are used in industrial applications and space propulsion. For a gridless ion source, a quasi-neutral plasma extends from the discharge region, through the acceleration region, into the beam. (An exception exists for a short distance of the acceleration region of an anode-layer source.) There may also be some overlap of the ion generation, ion acceleration, and beam regions in a gridless source. Such sources have been called both ion and plasma sources. For consistency herein, they are called “plasma sources.” In a gridless plasma source the acceleration can be electromagnetic—caused by the interaction of an electron current with a magnetic field, which establishes an electric field in a quasi-neutral plasma. The electron current that interacts with the magnetic field is supplied by a source of electrons at the exit of the source. This acceleration process is described in more detail in an article by Zhurin, et al., in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1.
The ion acceleration in a plasma source can also take place as the result of the expansion from a high plasma density to a low plasma density as it leaves the source. At the low background pressures assumed herein, the plasma potential and the density are related by the Boltzmann relation,ne=ne,oexp(Vp/Te),  (1)where ne,o is the reference plasma density where the plasma potential is defined as zero, Vp is the plasma potential at a density ne, and Te is the electron temperature in electron-volts. From Equation (1), the decrease in plasma density as the plasma leaves the plasma source results in a decrease in plasma potential that serves to accelerate the ions. The electrons in the beam are again supplied by the continuous plasma from the discharge region.
Yet another means of accelerating ions in a quasi-neutral plasma is described in U.S. Pat. No. 4,862,032—Kaufman, et al. As described therein, a gradient in magnetic field can interact with electrons to generate an electric field in a plasma, and the electric field will accelerate ions.
In a gridded source, electrons are present in the plasma of the discharge region, but they are excluded from the acceleration region between grids. The ion acceleration in such a source is electrostatic, i.e., caused by the voltage difference between the grids. The beam from a gridded ion source must be a quasi-neutral plasma (to avoid the mutual repulsion of a beam consisting only of positively charged ions), so electrons are added after electrostatic acceleration by an electron-emitting neutralizer. Gridded sources have been almost always been called “ion sources,” and that nomenclature is used herein. The means of extracting ions from a discharge plasma, accelerating them between electrically charged grids, and adding electrons to form a beam of quasi-neutral plasma are well understood by those skilled in the art and are described by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. It is also understood by those skilled in the art that, in the event of only grounded surfaces for the beam to impinge on, it is sometimes possible for the electrons added to the beam to come only from the secondary emission of ions striking grounded surfaces.
Beam nomenclature: If a source is called an “ion source,” the beam from it is usually called an “ion beam,” even though that beam satisfies quasi-neutrality and is a plasma. If a source is called a “plasma source,” the beam is usually called a “plasma” or “plasma beam,” although it has also sometimes been called an “ion beam.” Herein it is called simply a “beam,” which is defined as being comprised of energetic ions accompanied by sufficient electrons to make it a quasi-neutral plasma, regardless of whether the source is a plasma source or an ion source.
The particular sources described in the aforesaid article by Kaufman, et al., in the AIAA Journal use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, as described in U.S. Pat. No. 5,274,306—Kaufman, et al. for a capacitively coupled discharge, and U.S. Pat. No. 5,198,718—Davis, et al. for an inductively coupled discharge. These publications are incorporated herein by reference.
Plasma sources are described in the aforementioned U.S. Pat. No. 4,862,032—Kaufman, et al., and in the aforementioned article by Zhurin, et al., in Plasma Sources Science & Technology. The particular sources described in these publications use a direct-current discharge to generate ions. It is also possible for a gridless source to use a radio-frequency discharge, as described in U.S. Pat. No. 5,304,282—Flamm. These publications are also incorporated herein by reference. It should be noted that the aforesaid patent by Flamm uses the free expansion of a plasma for ion acceleration that was described previously.
The most common geometric configuration for either an ion (gridded) or plasma (gridless) source is one that generates a beam with a circular cross section. However, linear configurations, in which the cross section of the beam is greatly extended in one direction, have also been used. One such linear source is described by Wykoff, et al., in an article in Proceedings of the Eighth International Conference on Vacuum Web Coating, Las Vegas, Nev., Nov. 6-8, 1994, beginning on page 81. This publication is also incorporated herein by reference. In addition, beams with an annular cross section are described in the aforementioned article by Zhurin.
This patent is concerned with the generation of ions for a source, either ion or plasma, using an inductively coupled radio-frequency discharge. The beams from such sources have presented problems in that the distribution of energetic ions departed substantially from what was expected and/or needed. An ion source with a circular beam can be assumed to illustrate these problems. Such a source has a general axial symmetry and that symmetry would be expected to be reproduced in the beam. That is, while radial variations in ion current density might be expected, the beam would be expected to have symmetry about the axis of source symmetry. It is true that asymmetry can be introduced by such things as an asymmetric variation in spacing between ion-optics grids, but it is assumed that the design and construction of the ion source is carried out by those skilled in the art and the sources do not incorporate such obvious shortcomings.
To be more specific, the primary concern here is with those perturbations or departures from expectations associated with the inductor, comprised of multiple turns of high conductivity wire, that couples radio-frequency energy to the ion-generating discharge. There have been increasingly difficult requirements for precision in the control of beams from ion and plasma sources. At present, it is difficult to use the beams from these sources in many applications if the distributions of ion current density are not controlled to give reproducibility or beam symmetry within several percent. In some cases, that control results in a several-percent requirement for uniformity over most of the cross section of that beam.